I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for providing power control in a closed-loop communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users exist. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention, and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d also assigned to the assignee of the present invention, and is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user or mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
If the transmission power of signals transmitted by the base station to a mobile station is too high, it can create problems such as interfering with other mobile stations. Alternatively, if the transmission power of signals transmitted by the base station is too low, then the mobile station can receive multiple erroneous frames. Terrestrial channel fading and other known factors can affect the received power of signals transmitted by the base station. As a result, each base station must rapidly and accurately adjust the transmission power of the signals which it transmits to the mobile stations.
In a useful method for controlling the transmission power of signals transmitted by a base station, the mobile station transmits a signal or message to the base station when the power of a received frame of data deviates from a threshold or is received in error. In response to this message, the base station increases its transmission power of signals transmitted by the base station. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
A delay necessarily exists under the above closed loop communication system. The delay includes the time when the base station transmits the frame with inadequate power, when the mobile station receives the erroneous frame, when the mobile station detects the error (e.g., signal falls below the threshold or a frame erasure is declared), when the mobile station transmits an appropriate error message to the base station, and when the base station receives the error message and appropriately adjusts its transmission power. The communication system, typically, transmits packets of data as discrete frames. As a result, the communications system cannot rapidly compensate for transmission power fluctuations. Additionally, the mobile station could request too great of an increase in the forward link transmission power before recognizing such an increase due to the inherent delay described above.
The present invention solves problems of prior communication systems, and provides additional benefits, by allowing a mobile station or base station to anticipate the delays inherent in closed-loop communication systems. As noted above, mobile stations typically send quality or error messages over the reverse link channel to the base station, where the messages indicate the quality or power level of each frame or portions of the frame received in the forward link channel. If the mobile station detects a change in quality or power on the forward channel, the mobile station provides a message to the base station to request that the forward channel be appropriately increased or decreased.
In the present invention, the mobile station or receiver properly adjusts its forward link power level thresholds or measurements with which it compares incoming frames or portions of frames to reflect the quality or power level it anticipates receiving (after the aforementioned delay) in response to previously sent power adjustment commands. For example, the mobile station can recognize that at a given measurement time, two outstanding messages have not been executed by the remote transmitter (where each message indicates a corresponding increase of 1 dB). As a result, the mobile station can adjust its measurement threshold down by 2 dB to more closely correspond to future power adjustments. If the currently received frame or portion thereof is still below the readjusted threshold, then the mobile station sends out a new message to request a further increase in the power of the forward link channel. Conversely, if the received frame or portion of the frame is greater than the adjusted quality threshold, the mobile station does not request a further increase in power. The base station similarly sends messages over the forward link to the mobile station which indicate the quality or power level of the reverse link. As a result, the present invention can similarly be employed by the base station to properly adjust its reverse link quality level thresholds with which it compares incoming reverse link signals to reflect anticipated adjustments in the transmission power at the mobile station.
In a broad sense, the present invention embodies a communication system having a base station and at least one user station. A system user of the user station transmits to and receives from the base station communication signals. A method for controlling transmission signal power of transmitted communication signals for the base station includes the steps of: (a) transmitting a current communication signal; (b) receiving the current communication signal; (c) determining a quality level of the current communication signal; (d) adjusting a quality level threshold based on at least one prior power control message; (e) comparing the adjusted quality level threshold to the quality level of the current communication signal; (f) transmitting a current power control message based on the comparison; and (g) transmitting a new communication signal at an adjusted power level based on the current power control message.
The present invention also embodies a communication system having a base station and a user station. The base station transmits signals to and receives signals from the user station. The user station includes a receiver, a measurement circuit, a processor and a transmitter. The receiver receives a current received signal from the base station. The measurement circuit is configured to determine a level of the current signal. The processor is configured to adjust a measurement value based on at least one prior control message and compare the adjusted measurement value to the level of the current signal. The transmitter transmits a current control message to the base station to request that the base station transmit a new signal at an adjusted level based on the current control message.